The best answer to your VCS scaling issues is the one you gave in your question. Don't use CVS. I do agree with you though, SVN is the solution to no ones problems. There are plenty of highly scalable version control systems out there (Perforce, Rational are examples).
I think in general though you are going to find that clustered filesystems aren't going to provide the performance you are looking for, their main goals are availability. If you need to pick any clustered FS then I think you need to look into something like http://oss.oracle.com/projects/ocfs/ which is built for high performance database clustering. High performance databases, though, don't rely on flock or similar file locking mechanisms as CVS does, it just doesn't scale. You would need to add some sort of transactional distributed lock manager. CVS and high performance just don't fit in the same ballpark.
I do have a feeling though that you aren't trying to scale your source control system and you are trying to use CVS for something application specific. In that case I would suggest coding directly to RCS, and rolling your own lock manager. I would avoid the complication and expensive of distributed or clustered filesystems and concentrate on building a smarter app using some sort of distributed hash bucket approach.